Device for detecting magnetic fields and related detecting methods

ABSTRACT

A device for detecting magnetic fields, of the type comprising at least one element made of hard  5  magnetic material ( 12 ) and an element made of soft magnetic material ( 13 ) associated an element made of semiconductor material ( 11 ), electrodes ( 15 ) for forcing a current (I) in the semiconductor material ( 11 ), characterised in that the element made of hard magnetic material ( 12 ) and element made of soft magnetic material ( 13 ) are positioned in planar fashion on the element made of semiconductor material ( 11 ).

This application is the US national phase of international applicationPCT/IB2004/003056 filed 15 Sep. 2004 which designated the U.S. andclaims benefit of IT TO2003A000729, dated 23 Sep. 2003, the entirecontent of which is hereby incorporated by reference.

The present invention relates to a device for detecting magnetic fields,of the type comprising an element made of hard magnetic material and anelement made of soft magnetic material associated to an element made ofsemiconductor material, means for forcing a current in saidsemiconductor material.

According to the state of the art, to detect magnetic fields,magneto-resistive sensors are employed, i.e. devices whose resistance tothe passage of the electrical current varies with variations in themagnetic field whereto they are subjected. In particular, magneticsensors called AMR (Anisotropic Magneto Resistance) are known; they areusually obtained by means of a thin film of iron-nickel (permalloy),plated onto a silicon wafer and shaped in the form of a resistive strip.

The application of an external magnetic field determines a change in theorientation of magnetisation in the permalloy, making it not parallel tothe current that flows in the resistive strip and thereby increasingresistance. Said AMR sensors change their resistance by 2-3% in thepresence of magnetic fields. In order effectively to appreciate thechange in resistance, the AMR sensors are thus laid in such a way as toform a Wheatstone bridge.

However, the change in resistance is linked to the occurrence of themagneto-resistive effect, present in a limited quantity of materialssimilar to permalloy.

Moreover, such sensors are not easy to integrate and miniaturise andinvolve costly plating processes.

Also known are magnetic devices of the so-called ‘spin valve’ type,which provide a vertical stack of layers in which, in a simplifiedembodiment, between a layer of hard magnetic material and a layer ofsoft magnetic material is positioned a spacer layer made of dielectricor conductor material. The spin valve has a sharp reduction inelectrical conducibility if a magnetic field is applied in the oppositedirection to the magnetisation of the layer made of hard magneticmaterial.

However, said spin valve device has a complex structure to construct.

The object of the present invention is to provide a device for detectingmagnetic field able to have an increased sensitivity and a better easeof integration in simple plating processes at reduced cost.

According to the present invention, said object is achieved thanks to adevice for detecting magnetic fields and to a corresponding detectingmethod having the characteristics specifically set out in the claimsthat follow.

The invention shall be described with reference to the accompanyingdrawings, provided purely by way of non limiting example, in which:

FIGS. 1A and 1B show, in diagram form, two different operating states ofa device for detecting magnetic fields according to the invention;

FIG. 2 shows, in diagram form, a variant to the device for detectingmagnetic fields of FIG. 1A and

FIG. 3 shows, in diagram form, a first variant to the device fordetecting magnetic fields of FIGS. 1A and 1B;

FIG. 4 shows, in diagram form, a second variant to the device fordetecting magnetic fields of FIGS. 1A and 1B;

FIG. 5 shows, in diagram form, a third variant to the device fordetecting magnetic fields of FIGS. 1A and 1B;

FIG. 1A shows a device for detecting magnetic fields globally designatedby the reference number 10. Said device 10 comprises a substrate 11 madeof semiconductor material, e.g. silicon. A first strip 12, made of ahard ferromagnetic material, such as nickel-iron-cobalt or rare earthalloy, is plated onto the substrate 11 of semiconductor. Said firststrip 12 has an elongated parallelepiped shape, and a permanentmagnetisation is associated to it, represented in FIG. 1A by a vectorMP. Facing said first strip 12, along its shorter side, is positioned asecond strip 13, also with elongated parallelepiped shaped, made of asoft ferromagnetic material, such as iron or permalloy.

The first strip 12 and the second strip 13 therefore identify betweenthem a region 14 of substrate 11, which distances them from each other.

A current designated as I in FIG. 1A is forced by means of electrodes 15applied to the first strip 12 and to the second strip 13, so the path ofthe current I also includes the region 14 of semiconductor substrate 11.To the contacts 15 is associated a generator of the current I, as wellas means for measuring the variation in the resistance of the circuit,which are not shown in the figures.

FIG. 1A shows a first operating state of the device 10 in which anexternal magnetic field H is applied parallel and concordant to thedirection of permanent magnetisation and to the sense of the current Iin the semiconductor substrate 11.

When the external magnetic field H is concordant with the direction andsense of magnetisation of the first strip 12 of hard magnetic material12, the electrical resistance of the region 14 is low.

When the external magnetic field H has opposite sense, as shown in FIG.1B, and hence in the second strip of soft magnetic material 13 isinduced a temporary magnetisation MT with opposite sense to thepermanent magnetisation MP, the device 10 has the greatest electricalresistance. As FIG. 1B shows, the field lines, under the effect of theanti-parallel magnetic field H, tend to extend to regions surround theregion 14, influencing the mobility of the semiconductor substrate 11and thereby increasing total resistance.

The effect described above tends to be semi-superficial, i.e. it usuallyinvolves a layer with a depth of a hundred nanometres, with stronganalogies with two-dimensional electronic gases. In this regard, it ispossible to construct the substrate 11 in the form of hetero-structurewith two-dimensional bordering, to improve the characteristics ofsensitivity of the device for detecting magnetic fields 10.

GaAs and AlGaAs based hetero-structures can be used in which the variousepitaxial layers continuously vary their stoichiometry. For example, onecan start by plating a layer of GaAs and, during the growth, lay Al inincreasing concentrations until reaching the desired final stoichiometryof AlGaAs. The stoichiometric ratios of As and Ga can also change duringgrowth.

The device for detecting magnetic fields 10 shown in FIGS. 1A and 1B cantherefore comprise a substrate 11 constituted by a semiconductormaterial like Si, Ge, InSb, Hg(Cd)Te, InAs, TiC, GaAs, SiC, GaP, GaN, ora AlGaAs/GaAs hetero-structure or a combination of said semi-conductormaterials and metal parts of one or more metals. In a preferred versionof the invention, a semiconductor with high electronic mobility such asInSb is used.

The semiconductor constituting the substrate 11 can be laid onto anyother substrate, for example silicon or glass, by continuous or pulsedelectrical plating, electrochemical methods, simple precipitation,centrifuging, thermal evaporation or electron beam, simple or magnetronsputtering, CVD, PECVD, serigraphy.

The thickness of the semiconductor substrate 11 can range from onenanometre to some hundreds of micrometres.

On the semiconductor substrate 11 the first strip of hard magneticmaterial can be obtained from a hard magnetic alloy such as CoNi80Fe20,whilst the second strip 13 of soft magnetic material can be made, by wayof example, of permalloy. Clearly, the person versed in the art mayalternatively use many other different materials, able to have arespectively hard or soft magnetic behaviour, such as ferromagneticalloys with different stoichiometric compositions, Ni, Fe, Co, or rareearth metals. These materials can be plated by plating methods such ascontinuous or pulsed electroplating, electrochemical methods, simpleprecipitation, centrifuging, thermal or electron beam evaporation,simple or magnetron sputtering, CVD, PECVD.

FIG. 2 shows a device for detecting magnetic fields 20, variant to thedevice 10 of FIGS. 1A and 1B, in which the first strip 12 and the secondstrip 13 are obtained by means of multiple plating of metal layers, asan alloy or as a sandwich of layers. Moreover, instead of the region 14of semiconductor, there is a layer of polymer 24 which incorporatesclusters of metallic or semiconductor atoms. This polymer can beinsulating, conjugated or conductor, or also a light emitter. The metalclusters can also be plated directly onto the region 14 shown in FIGS.1A and 1B, without incorporating in the polymer layer 24, with methodssuch as continuous or pulsed electroplating, electrochemical methods,simple precipitation, centrifuging, thermal or electron beamevaporation, simple or magnetron sputtering, CVD, PECVD or anotherapparatus for the formation and controlled laying of clusters.

In the fabrication of the devices 10 and 20 the first strip 12 and thesecond strip 13 can be geometrically defined using photolithographytechniques, or by means of electronic or ionic beam.

The first strip 12 and the second strip 13 must be separated by adistance which can vary from a few angstrom and hundreds of micrometres.

The dimensions such as thickness and width of the metal strips can alsovary from one nanometre to hundreds of micrometres.

In a possible variant, the magnetisation state of the first strip 12 andof the second strip 13 can be induced by means of permanent magnets orby the passage of electrical current on two tracks that are orthogonalto the strips. Said electrical tracks for the magnetisation of themetallic strips such as the first strip 12 and the second strip 13 canbe obtained over the strips and also obtained by photolithography. Theelectrical tracks for the magnetisation of the metal strips areelectrically insulated from the metal strips.

The electrical insulation between the metal strips and the magnetisationtracks is obtained by plating any layer of electrically insulatingmaterial, for example oxide or ceramic material.

Said first strip 12 and second strip 13 in a possible alternativeembodiment can be plated in two windows dug by etching in thesemiconductor substrate 11. The etching process can be executed byphotolithography, by means of electronic or ionic beam ornano-imprinting.

FIG. 3 shows a device for detecting magnetic fields 30 which constitutesand additional embodiment of the device according to the invention.

Said device 30 comprises, plated onto the semiconductor substrate 11,two first strips 32 made of hard ferromagnetic material, interspersed bya second strip 33 of soft magnetic material, so between the two firststrips 33 and the second strip 34 are defined two regions 34 of freesubstrate 11.

The second strip 33 constitutes an insulated platelet which provides thesensitive element to the external magnetic field and can have anygeometric shape and dimensions. The sensitivity of the device depends onthe geometric parameters of said platelet or second strip 33.

FIG. 4 shows a device for detecting magnetic fields 40 which constitutesan additional embodiment of the device according to the invention, inwhich the current flows in orthogonal direction relative to the axis ofmagnetisation. For this purpose, electrodes 45 for applying the currentI to the substrate 11 are plated along an axis that is orthogonal to theone defined by the first strip 12 and by the second strip 13.

The solution described above enables to achieve considerable advantageswith respect to prior art solutions.

The device of the invention, using a planar geometry, is advantageouslysimple to construct, by means of not very expensive technologies, whilstobtaining a high sensitivity.

Naturally, without altering the principle of the invention, theconstruction details and the embodiments may vary widely from what isdescribed and illustrated purely by way of example herein, withoutthereby departing from the scope of the present invention.

A device for detecting magnetic fields comprising an element made ofhard magnetic material and an element made of soft magnetic materialassociated to an element made of semiconductor material, means forforcing a current in said semiconductor material, where said elementmade of hard magnetic material and element made of soft magneticmaterial are positioned in planar fashion on said element made ofsemiconductor material can be used as a magnetic field sensor ormagnetic switch, as a sensor of electromagnetic radiation, as an emitterof electromagnetic radiation, as a photovoltaic cell, and as athermo-photovoltaic cell.

1. A device for detecting magnetic fields, of the type comprising atleast one element made of hard magnetic material and an element made ofsoft magnetic material associated to an element made of semiconductormaterial, means for forcing a current in a region of said semiconductormaterial, wherein said element made of hard magnetic material and saidelement made of soft magnetic material and said means for forcing acurrent are respectively positioned in a planar geometry on said elementmade of semiconductor material, and said element made of hard magneticmaterial is configured for applying a permanent magnetization in saidregion of semiconductor material parallel to the plane of saidsemiconductor material.
 2. A device as claimed in claim 1, wherein saidelement made of hard magnetic material is distanced relative to saidelement made of soft magnetic material to define said region ofsemiconductor material.
 3. A device as claimed in claim 1, wherein saidregion of semiconductor material comprises polymeric material thatincorporates clusters of metallic or semiconductor atoms.
 4. A device asclaimed in claim 3, wherein said polymeric material is plated onto saidregion of semiconductor material of the substrate.
 5. A device asclaimed in claim 2, wherein comprises at least two elements made of hardmagnetic material positioned around an element of soft magnetic materialand defining at least two semiconductor regions.
 6. A device as claimedin claim 1, wherein said element made of hard magnetic material and/orsaid element made of soft magnetic material comprise multiple layers. 7.A device for detecting magnetic fields as claimed in claim 1, whereinsaid at least one element made of hard magnetic material and one elementmade of soft magnetic material are plated in windows dug by etching intothe semiconductor substrate.
 8. A device as claimed in claim 1, whereinthe means for forcing a current in said semiconductor material comprisemetallic contacts.
 9. A device as claimed in claim 8, wherein saidmetallic contacts are positioned in substantially perpendicular fashionrelative to an axis defined by said at least one element made of hardmagnetic material and one element made of soft magnetic material.
 10. Adevice as claimed in claim 1, wherein said at least one element made ofhard magnetic material comprises a hard ferromagnetic alloy, and theelement made of soft magnetic material comprises permalloy.
 11. A deviceas claimed in claim 1, wherein said substrate is obtained by means of atleast one semiconductor selected among Si, Ge, InSb, Hg (Cd) Te, InAs,TiC, GaAs, SIC, GaP, GaN.
 12. A device as claimed in claim 1,characterised in that wherein said substrate is obtained in the form ofhetero-structure with two-dimensional bordering.
 13. A method fordetecting magnetic fields, of the type providing at least one elementmade of hard magnetic material, associated to a permanent magnetisationand one element made of soft magnetic material associated to a temporarymagnetisation induced by an external magnetic field wherein it furthercomprises the operations of: arranging the element of hard magneticmaterial and the element of soft magnetic material in such a way thatthe respective temporary magnetisation and permanent magnetisation liesubstantially in one plane; forcing a current in a region ofsemiconductor material in said plane located between said element ofhard magnetic material and element of soft magnetic material; measuringthe value of resistance in said region of semiconductor materialaccording to the values assumed by an external magnetic field.
 14. Adevice as claimed in claim 10, wherein the hard ferromagnetic alloy isCoNi80Fe20.
 15. A device as claimed in claim 12, wherein thehetero-structure is a GaAs/AlGaAs hetero-structure.